Catalyst for methanation of carbon oxides, preparation method of the catalyst and process for the methanation

ABSTRACT

Disclosed is a catalyst for methanation reaction producing methane with high conversion by reaction of hydrogen with carbon dioxide, or a gas mixture of carbon dioxide and carbon monoxide, or a gas mixture containing these compounds as the main components. The catalyst is prepared by the steps of mixing (A) aqueous zirconia sol with salts of (B) stabilizing element(s), which is selected from the group consisting of Y, La, Ce, Pr, Nd, Sm, Gd, Dy, Ca and Mg, and (C) iron group element(s), drying and calcining the mixture to obtain a catalyst precursor, and subsequent reduction of the precursor. The catalyst comprises, by atomic %, A: 18-70%, B: 1-20% and C: 25-80% based on the elemental states of the metals. The catalyst is characterized by multiple oxide of tetragonal zirconia structure, in which not only the stabilizing element(s) but also a part of the iron group element(s) is incorporated, and on which the iron group element(s) in the metallic state is supported.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 12/184,493, filed Aug. 1, 2008, which claims priority toJapanese Patent Application No. 2007-203653, filed Aug. 3, 2007, andJapanese Patent Application No. 2008-187970, filed Jul. 18, 2008, whichare hereby incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

Field in the Industry

The present invention concerns a catalyst for methanation or formationof methane by reaction of hydrogen with carbon dioxide, a mixture ofcarbon monoxide and dioxide, or a mixed gas containing them as the maincomponents. The invention also concerns method of preparing the catalystand the process for the methanation using the catalyst.

STATE OF THE ART

Global warming due to carbon emissions as a result of combustion offossil fuels is getting serious, and ways for decreasing the emissionhave been sought. As one of the countermeasures, the method of producingmethane by reaction of carbon dioxide with hydrogen is expected toprevent global warming and supply fuel. Also, hope is placed on thetechnology of producing fuel gas with high combustion energy bymethanation from low combustion energy gas mixture of hydrogen, carbonmonoxide and carbon dioxide formed by gasification of coke, coal,biomass, activated sludge, and so on.

To date Raney nickel and catalysts supported on alumina or silica havebeen examined as the catalysts for methane production by contactreaction of hydrogen with carbon dioxide, carbon monoxide or theirmixture. However, because reaction rates with these catalysts are soslow that the reaction should be carried out under a high pressure.

The inventors discovered the fact that ribbon-shaped amorphous alloys,prepared by rapid quenching of the liquid state, consisting of an irongroup element such as Ni and Co, and a valve metal such as Zr, Ti, Nband Ta are effective catalysts for methane synthesis, and disclosed asJapanese Patent Disclosure Nos. 10-43594, 10-244158 and 10-263400. Theselectivity to methane formation achieved by these catalysts is almost100% and the conversion rate is very fast at ambient pressure. However,the process for producing the catalyst alloys, rapid quenching from theliquid state, is not suitable for mass production, and the applicablesystems of ribbon-shaped catalysts are limited.

On the basis of the above-mentioned discovery the inventors developed amethod of producing powder-shaped catalyst, mass-production of which ismuch easier than the ribbon-shaped catalysts, and disclosed as JapanesePatent Disclosure No. 2000-254508. This catalyst is composed of Nisupported on tetragonal ZrO₂, and prepared by calcination of tetragonalzirconia-type oxides including tetragonal structure-stabilizing elements(Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Eu, Mg and/or Ca), followed byimpregnation of Ni and/or Co and final reduction to form metallic Niand/or Co.

The catalyst disclosed in Japanese Patent Disclosure No. 2000-254508includes the following embodiments:

(1) A catalyst for carbon dioxide methanation, which comprises Ni and/orCo supported on tetragonal zirconia-type supporter containing astabilizing element or elements selected from the group consisting of Y,La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Eu, Mg and/or Ca, wherein the ratio ofZr to the sum of Zr and the stabilizing element(s) is 85 atomic % ormore and wherein the ratio of Ni and/or Co to the total metallicelements is 0.05-0.5.

(2) The method of preparing the catalyst for carbon dioxide methanation,which comprising the steps of forming tetragonal zirconia-type supporterby addition of one or more of salts selected from the group consistingof Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Eu, Mg and Ca to aqueous zirconiasol under stirring, drying and calcining, and immersing the tetragonalzirconia-type supporter in a solution of a salt or salts of Ni and Co,drying calcining, and final reduction.

As the result of further research the inventors discovered the factthat, instead of impregnation of Ni and/or Co to tetragonalzirconia-type oxide previously prepared as described in Japanese PatentDisclosure No. 2000-254508, preparation of oxide containing all thecomponents necessary for the catalyst and subsequent reduction will givea catalyst with better performance, and thus, achieved the newinvention.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a catalyst for carbondioxide methanation with high activity and selectivity even at atemperature of 250° C. or lower, which is useful also for production ofmethane by the reaction of the gas mixture of carbon dioxide, carbonmonoxide and hydrogen utilizing the above-noted novel method ofproducing the catalyst.

The catalyst according to the present invention is the catalyst formethanation reaction by hydrogenation of carbon dioxide, mixture ofcarbon dioxide and carbon monoxide, mixed gas containing these gases asthe main component, which comprises, the total being 100% based on themetals in the elemental states:

A) a tetragonal zirconia structure-stabilizing element, one or moreselected from the group consisting Y, La, Sm, Ce, Pr, Nd, Gd, Dy, Ca andMg: 1-20 atomic %;

B) Zr for composing tetragonal zirconia supporter: 18-70 atomic %; and

C) an iron group element bearing the catalytic activity: 25-80 atomic %;

and in the catalyst the iron group element is supported on the oxide ofthe tetragonal zirconia structure in which not only the stabilizingelement but also a portion of the iron group element is incorporated tostabilize the tetragonal zirconia structure.

BRIEF EXPLANATION OF THE DRAWING

The attached single drawing is a diagram showing chemical composition ofthe catalyst according to the invention.

DETAILED EXPLANATION OF THE PREFERRED EMBODIMENTS

The method of preparing the catalyst according to the present inventionis the method of preparing the above-described catalyst for themethanation reaction, which comprises the steps of mixing hydrosol ofzirconia, aqueous solution of salts of the stabilizing element(s) andthe iron group element(s) in the ratio satisfying the above-notedconditions for the catalyst composition, and subsequent reducingtreatment so as to realize the catalyst structure of supporting the irongroup element on the oxide of the tetragonal zirconia structure in whichnot only the stabilizing element but also a portion of the iron groupelement is incorporated in the crystal structure.

The method of preparing the catalyst according to the present inventionis characterized by employing, not the two step method as disclosed inJapanese Patent Disclosure No. 2000-254508 comprising formation of thesupporter with tetragonal zirconia structure and impregnating aqueoussolution of the salt of the active metal followed by reduction, but theone step method by mixing all the necessary components in the form ofaqueous solutions, drying and calcinations followed by reduction.

According to the above-described one step method, in the process ofdrying to calcinations, mixture of iron group element oxide and multipleoxide containing not only the stabilizing element(s) but also the irongroup element(s) is formed. The oxide of the iron group element isreduced during the subsequent reduction, and as the result, the catalysthaving the structure where the iron group element of the metallic stateis supported on the multiple oxide of tetragonal zirconia structure isformed. In this manner after the reduction a portion of iron groupelement(s) remains in the supporter, which is multiple oxide oftetragonal zirconia structure, in addition to the stabilizingelement(s).

It is possible to form catalyst grains of maximum 3 mm diameter bymixing the mixture of zirconia hydrosol, a salt of the stabilizingelement and a salt of the iron group element with silica, alumina orother oxide particles as the core of the catalyst and a binder such assilicate, titanate, aluminate and zirconate, and subsequent drying andcalcination. The core particles and the binders may be used in analternative way such as mixing core particles and a binder with alreadyprepared catalyst powder followed by calcination.

The catalyst according to the present invention is explained in detail.The stabilizing elements, Y, La, Ce, Pr, Nd, Sm, Gd, Dy, Ca and Mg arethe constituents to stabilize the tetragonal zirconia structure duringcrystallization of zirconia from sol. Thus, the stabilizing element(s)is necessary in an amount of 1 atomic % or more, but excess addition isharmful to the catalyst activity due to formation of its own oxide. Theaddition of the stabilizing elements should be at highest 20 atomic %.

Zr is the essential element to form the supporter of tetragonal zirconiastructure, and it is necessary to add 18 atomic % or more. However, iftoo large amount is added, concentration of the iron group element(s)will not be so high as sufficient to the catalyst activity, and hence,70 atomic % is set as the maximum content.

The iron group element is the catalytically, active component, and 25atomic % or more is necessary to exist in the catalyst. However, excessaddition of the element leads to coagulation causing lowered dispersionwith consequent decrease in the activity. Thus, addition of the elementshould be 80 atomic % or less. As the iron group element Ni mustnecessarily be used. Apart of Ni can be replaced with the other irongroup element(s), Fe and/or Co. In that case, the atomic ratio of Ni tothe sum of iron group elements must be 0.6 or higher.

High activity and durability of the catalyst according to the inventioncan be explained as follows. The stable phase of zirconium oxide ismonoclinic. However, the active component, the iron group elementsupported on the tetragonal zirconia which is stabilized by inclusion ofstabilizing element(s) exhibits particularly high activity formethanation of carbon dioxide and carbon monoxide. This has beenexplained in Japanese Patent Disclosure No. 2000-254508.

The present invention is the fruits of further development, whichutilizes the facts that calcination of mixture of aqueous zirconia sol,solutions of the salts of stabilizing element(s) and the iron groupelement(s) causes formation of mixture of oxide(s) of the iron groupelement(s) and multiple oxide of tetragonal zirconia structureconsisting of oxidized zirconium, and stabilizing element(s), in which apart of the oxidized iron group element(s) is incorporated, and that,when the mixture of oxides is subjected to hydrogen treatment, themultiple oxide containing zirconium and the stabilizing element(s) withhigh affinity to oxygen remains in the oxidized state and only the oxideof the iron group element(s) directly contacted to hydrogen is reducedto the metallic state. Thus, the catalyst in which catalytically activeiron group element(s) in the metallic state is finely dispersed on themultiple oxide of tetragonal zirconia structure is obtained.

In such the catalyst, even if the metallic iron group element(s) on thecatalyst surface is lost by abrasion due to passage of reaction gases,the iron group element(s) in the oxidized state emerges from the innerparts and is readily reduced by hydrogen in the reaction gases, andthus, acts as the active substance. Hence, the state of high activity ofthe catalyst due to finely dispersed active sites on the tetragonalzirconia can be preserved.

Procedures for preparing the catalyst according to the present inventionare as follows: one or more salts of the stabilizing element(s) selectedfrom the group consisting of Y, La, Ce, Pr, Nd, Sm, Gd, Dy, Ca and Mg ismixed with aqueous zirconia sol, and to the mixture, an aqueous solutionof Ni salt, or Ni salt with Fe salt and/or Co salt is added. Theresulting mixture is concentrated by heating under stirring to dry, andcalcined at about 500° C. in air to form mixture of the multiple oxideof the tetragonal zirconia structure and the oxide(s) of the iron groupelement(s). Reduction treatment of the mixture of the oxides in hydrogenstream at about 300° C. gives the catalyst in which the metallic irongroup element(s) is supported on the multiple oxide of tetragonalzirconia structure.

Table 1 shows composition of the catalyst of the present invention.

TABLE 1 Elements composing Elements acting as tetragonal zirconia activesite in supporter catalysis Composition Zr Y, La, Ce, Pr, Iron groupelement: Nd, Sm, Gd, Dy, Ni or Ni with Fe and/or Ca, Mg Co (The ratio ofNi to the sum of iron group elements must be 0.6 or higher) Content18-70 1-20 25-80 atomic % atomic % atomic %

The catalyst according to the present invention in which metallic stateiron group element(s), Ni or Ni with Fe and/or Co is supported on themultiple oxide of tetragonal zirconia structure exhibits methaneselectivity of neary 100%, and the reaction equilibrium is extremelyproduct-sided even under a normal pressure. Thus, problem of complicatedprocedures of repeating the reaction under high pressures after removalof impurities from the reaction mixture to recycle the unreactedmaterials is solved. Now, complicated system and equipment forcirculation of reactants is no longer necessary, and conversion of a gasmixture of hydrogen with carbon dioxide, carbon monoxide or theirmixture to methane is rapidly performed under ambient pressure insteadof high pressures.

On the catalyst of the present invention an ideal situation can berealized: poisonous carbon monoxide in the gas mixture of carbonmonoxide, carbon dioxide and hydrogen is preferentially and selectivelyconverted to methane, and thereafter, carbon dioxide is converted tomethane with the remaining hydrogen. Furthermore, the production of thecatalyst according to the present invention is easy.

During the methanation of carbon monoxide, the iron group element in theoxidized state, particularly, nickel contained in the tetragonalzirconia lattice may be sometimes selectively lost by the reaction withcarbon monoxide. However, even if oxidized nickel is lost, thetetragonal zirconia structure remains as stabilized by the presence ofthe stabilizing element(s), and hence, the high catalytic activity isretained without being changed.

EXAMPLES Example 1

To 15.0 g of aqueous zirconia sol “Zr30AH” (Nissan Chemical) (30 wt. %Zr, pH 4.0), 1.58 g of Sm(NO₃)₃.6H₂O was added, and stirring was carriedout until creamy sludge was formed. A nickel nitrate solution wasprepared by dissolving 19.806 g of Ni(NO₃)₂.6H₂O in 20 ml of water. Thenickel nitrate solution was mixed with the zirconia sol-samarium nitratesludge under stirring. After standing still for 1 hour, the mixture waskept in a muffle furnace at 150° C. for 4 hours to remove water and dry.Then, the dry substance was calcined by heating at 500° C. for 8 hoursto obtain calcined black solid, the weight of which was 10.3 g. Theblack solid was pulverized in an agate mortar and the resulting powderwas classified with a sieve of 100 mesh to give the catalyst precursor.The metallic fraction of Ni in the catalyst precursor was 0.625 and thecomposition was written as Ni_(0.625)(Zr_(0.692)Sm_(0.108)O_(1.946))_(0.375). The tetragonal polymorph ofzirconia was identified by X-ray diffraction with Cu-Kα radiation. Thespecific area of the precursor was 80.1 m²/g.

A quartz tube of inner diameter 15 mm was used as a reactor, and 5.0 gof the above-obtained catalyst precursor was inserted on quartz wool inthe quartz tube. The catalyst precursor in the reactor was reduced byheating at 300° C. in an electric furnace under hydrogen stream for 2hours. The catalyst was thus obtained.

A gas mixture, in which the volume ratio of carbon dioxide to hydrogenwas 1/4, was passed through the reactor containing the catalyst at 250°C. After the reaction the gas was passed through a cold water trap toremove water and then analyzed by gas chromatography. The analysis ofmethane and unreacted carbon dioxide and hydrogen revealed that theconversion of carbon dioxide on 1 g of the oxide mixture of the catalystprecursor at the gas flow rate of 5.4 L/h was 82.2%, and that thereaction product was only methane showing the 100% methane selectivity.

Control Example

Carbon dioxide conversion of the catalyst having the composition ofNi_(0.5) (Zr_(0.833)Sm_(0.167)O_(1.92))_(0.5), i.e., metallic fractionof Ni in the catalyst being 0.5, which was one of the catalystsdescribed in Japanese Patent Disclosure No. 2000-254508, was reported tobe 52.6% on 1 g of the Ni-impregnated catalyst precursor at the gas flowrate of 4 L/hour at 250° C.

Example 2

Catalysts were prepared by the same procedures as Example 1 usingvarious compositions of Ni, stabilizing element(s) and Zr. Carbondioxide conversion was examined in regard to the respective catalysts bythe same procedures as those of Example 1. The results are shown inTable 2. All the catalysts showed high conversion at 200° C. and 250°C., indicating the excellent performance of the catalysts.

TABLE 2 Composition of the Metal CO₂ Elements Atomic % Conversion %Stabilizing 200° 250° No. Ni Zr Element C. C. 2 25 62.5 Y 12.5 39.2 59.63 50 41.7 Y 8.3 45.5 68.9 4 70 25 Y 5 38.1 56.1 5 25 62.5 La 12.5 42.167.1 6 57 33 La 10 59.5 73.2 7 25 62.5 Ce 12.5 40. 1 66.7 8 50 41.7 Ce8.3 50.6 69.7 9 25 63 Pr 12 49.8 66.3 10 57 33 Pr 10 58.7 72.5 11 25 63Nd 12 50.2 67.1 12 57 33 Nd 10 60.1 73.2 13 25 56.3 Sm 18.7 49.9 66.6 1429.5 69 Sm 1.5 49.5 65.4 15 41.6 52.1 Sm 6.3 64.3 78.8 16 50 41.7 Sm 8.471.3 82.2 17 62.4 33.5 Sm 4. 1 72.8 82.2 18 62.5 19 Sm 18.5 59.8 70.1 1979.5 18.3 Sm 2.2 43.2 73.1 20 25 63 Gd 12 49.5 66.2 21 57 33 Gd 10 60.172.2 22 25 63 Dy 12 50.7 66.3 23 57 33 Dy 10 59.8 71.5 24 47.5 33 Mg19.5 60.1 71.5 25 54 40 Mg 6 65.2 77.3 26 54 40 Ca 6 66.1 78.8 27 54.126.1 Ca 19.8 60.1 72.2 28 56.2 30.1 Ca 13.7 65.0 78.0 29 62 34 Ca 4 73.882.3 30 62 33 Sm 3 Ca 2 72.5 82.0 31 60 34 La 2 Sm 2 Ca 2 73.3 82.2 3261 33 Y 1 La 1 Ce 1 71.2 81.0 Sm 1 Ca 1 Mg 1 33 60 34 Nd 1 Sm 1 Gd 173.5 82.2 Dy 1 Ca 1 Mg 1

Example 3

Catalysts were prepared by adding Sm (NO₃)₃.6H₂O or Ca (NO₃)₂ and,Ni(NO₃)₂ together with one or two of Co(NO₃)₂ and Fe(NO₃)₃ to a mixturesof aqueous zirconia sol, and subsequent drying, calcination andreduction. The carbon dioxide conversion of the catalysts was examinedby the same procedures as Example 1. The results are shown in Table 3.All the catalysts showed high conversion to methane at 200° and 250° C.

TABLE 3 Composition of Metallic Elements Conversion in the CatalystsAtomic % of CO₂ % No. Ni Fe Co Zr Sm Ca 200° C. 250° C. 34 30 20 41.88.2 49.7 63.1 35 30 20 41.8 8.2 53.5 65.4 36 30 10 10 41.8 8.2 46.5 59.737 36 24 36 4 59.8 71 38 36 24 36 4 60.1 72.1 39 36 12 12 36 4 58.0 70

Example 4

Catalyst were prepared as in Example 1 by adding Sm(NO₃)₃.6H₂O, Ca(NO₃)₂or Ni(NO₃)₂ mixed in various ratios to aqueous zirconia sol, andsubsequent drying, calcination and reduction. A reactant gas whichconsists of, in molar %, CO:15.5%, CO₂:14.5% and H₂:70% was passedthrough the reactor tube, in which the catalysts were kept at 300° C.Flow rate of the reactant gas is 5.4 L/Hr per catalyst 1.0 g (based onthe precursors). The outlet of the reactor was led to a gaschromatography to analyze the reaction gas, and the conversion wasdetermined from the ratio of the introduced gases and the unreactedgases.

The results are shown in Table 4. It was ascertained that all thecatalysts, at 300° C., first converted whole the carbon monoxide tomethane, and then, the remaining hydrogen was used for converting carbondioxide to methane.

TABLE 4 Composition of Metallic Elements in the Conversion CatalystsAtomic % at 300° C. % No. Ni Zr Sm Ca CO CO₂ H₂ 40 25 62.5 12.5 100 2688 41 30 58 12 100 28 89.6 42 50 41.7 8.3 100 28.5 90 43 70 25 5 100 2788.8 44 62 34 4 100 28.5 90

We claim:
 1. A process for methanation of carbon oxides, which comprisescontacting a mixed gas of carbon dioxide, mixture of carbon dioxide andcarbon dioxide, or a mixture containing at least one of them as maincomponents and hydrogen with a catalyst consisting of, based on themetals in the elemental state: A) Zr for providing tetragonal zirconiasupporter: 18-70 atomic %, B) one or more of tetragonalzirconia-stabilizing element selected from the group consisting of Y,La, Ce, Pr, Nd, Sm, Gd, Dy, Ca and Mg (in case of two or more are used,in total): 1-20 atomic %, and C) at least one of the iron group elementsbeing supported on the tetragonal zirconia supporter and acting as theactive site: 25-80 atomic %; wherein the stabilizing element or elementsand a portion of the iron group element or elements are incorporated inthe tetragonal zirconia supporter; and wherein the iron group element orelements not incorporated in the supporter are supported on thesupporter in the metallic state.
 2. The process of claim 1, wherein: theiron group element acting as the active site is Ni.
 3. The process ofclaim 1, wherein: the iron group elements acting as the active site areNi and one or both of Co and Fe, provided that Ni shares 0.6 or morebased on the atomic ratio.